The present invention relates to tools for applying clamping fasteners, such as a lock screw and/or a lock nut, to a surgical implant, for example, to the head of a spinal anchor screw, or to a hook, to secure a linking member, such as a rod or cable. The linking member is also clamped to one or more other anchors, for example to stabilize or correct the spine.
A great number of such anchor bolt or screw assemblies are known. A typical procedure or operation may involve the placement of pedicle screws at various positions in hard tissue. Each screw directly or indirectly (e.g., via an offset plate) holds a head or cap structure into which the linking member fits to connect the anchor screw to other anchor screws situated remotely on adjacent vertebrae, and thereby operates to adjust or stabilize a region of the spine.
The anchor screw head or cap structure generally has a slot to accommodate the linking member, and has some mechanism for closing the top of the slot and clamping the rod or other linking member therein. Typically this is done in several stages, by first setting the linking member within the slotted head, for example capturing the connecting member with a cap or a lock screw that closes the receiving slot, and thus allowing the surgeon, in effect, to roughly align and/or contour the linking member; and then adjusting and effecting final clamping of the connecting member (e.g., with a lock nut or other fasteners).
Among the known constructions of this sort, one common construction provides the anchor screw with a slotted head that is internally threaded to receive an internal set screw, and the set screw is tightened down to initially fix the rod in place. The head may also be externally threaded such that a nut threaded about the outside provides additional circumferential strength, more forcefully clamps the rod, and firmly locks the internal set screw. Yet, other constructions involve a twist-on or slide-on cap that also accommodates a central set screw. The cap is first placed on the slotted head to capture the linking member, and the set screw is then tightened down. It is also possible to provide an external nut about the set screw to allow the set screw itself to be more firmly locked in such a cap member.
All such implanted anchor assemblies present the problem that the slotted head member must initially be open for receiving the connecting member, and the process of fitting or adjusting the rod requires tightening to be performed in several steps. Thus the process of capturing, adjusting and clamping the connecting member in the anchor assembly, which requires manipulation of several small components, presents a substantial risk that these components may be dropped or misplaced. Furthermore, the threading of one or more fasteners onto the anchor member, which often must be done deep in a wound, or at an unusual angle, with little room for manipulation, can be problematic and great care must be exercised to avoid cross-threading any of the fasteners.
It will be appreciated that during spinal surgeries utilizing a spinal rod system with pedicle screws and/or vertebral hooks, typically several compression and distraction techniques are carried out to compress or to distract the spinal column. A compression maneuver, for example, is done by positioning a compressor against two adjacent implants that are locked in place. The compressor is squeezed together. At the same time, the clamping fastener fixing the linking member to one of the implants is loosened, which allows the adjacent implants to be forced closer to each other along the rod. The fastener at that implant is then re-tightened. This maneuver results in incremental correction of the spine (i.e., it creates lordosis or kyphosis), and the surgeon may loosen and tighten the same implant several times before the spinal column is totally corrected.
One common set of hardware for such operations is the MOSS(copyright) Miami System sold by Depuy AcroMed, Inc., of Raynham, Mass., which utilizes an inner screw and an outer nut, thus a dual clamping mechanism, to secure a connecting member such as a rod to the implant e.g., anchor screw. The inner screw is typically set to temporarily lock the position of the implant on the rod during compression and distraction techniques. The inner screw achieves just enough force at low torque to lock the implant in place. If one were to use the outer nut, which is larger in diameter, it would clamp with greater force but would require the surgeon to exert a much higher torque to tighten and loosen the fasteners during the compression and distraction maneuvers. Generally, once suitable compressions and distractions have been carried out repositioning the anchor assemblies along the rod linking member, final tightening is effected by advancing the outer nut sufficiently so that it also contacts the connecting rod.
Typically these steps have required the use of several tools to install and tighten the clamping fasteners, e.g., the clamp screw and clamp nut. Moreover the process poses multiple risks of dropping or misplacing a fastener, or of misaligning (cross-threading) a fastener in the course of the multi-step procedure.
It would therefore be desirable to provide an enhanced tool for installing clamping fasteners on spinal implants and anchor assemblies.
One or more of these and other desirable ends are achieved in accordance with the present invention by a fastener cartridge that holds one or more threaded fastening elements and applies the element(s) to an anchor in alignment. The cartridge may simultaneously and coaxially hold both an inner and an outer threaded clamping fastener, coordinating the application of both elements, e.g., a set screw and a nut, to the head of an anchor screw. The cartridge has an outer wall forming a box wrench configured to both hold and turn the outer clamping fastener, while an inner shaft, having a cross sectional shape such as an Allen, star or other internal wrench, extending in the cartridge is configured to centrally hold and turn an inner fastener or set screw. A spring detent or other friction member may be arranged about the inner face of the outer wall to provide a holding force that gently retains the clamp nut therein. Another spring, detent or gripping element may be mounted on the central shaft to provide a similar holding force retaining the set screw on the central torquing shaft. In each case, the retaining force prevents the nut or screw from passively falling out of the cartridge, but allows it to advance as the cartridge turns during fastener installation.
In one embodiment, an axially offset central region is provided at the floor of the cartridge to position the inner clamp screw ahead of the outer clamp nut. This assures that the inner member is threaded onto the anchor head structure before the outer clamp nut, or starts threading a defined distance or number of threads before the nut advances. This central region may be spring loaded in an axial direction, allowing the set screw to float axially until its threads engage. In a method of use, when neither the inner and outer fasteners are initially matched with the starting thread of their opposed anchor, the cartridge may be rotated in a direction opposite the thread direction (e.g., counter clockwise) until the inner fastener drops into alignment, then turned clockwise to engage and advance the inner screw.
In further or other embodiments of the cartridge, the cartridge may have an elongated body that can be loaded with plural pairs of inner and outer clamp fasteners. In this case, the cartridge may include a spring-loaded pusher mechanism, and the fasteners may be loaded into a recess such that the pusher mechanism successively advances each pair of fasteners forward toward the distal face of the cartridge as the preceding pair is threaded onto an implant and leaves the cartridge. The cartridge itself, in either the single-pair or the multiple-pair embodiments, may be used as a stand-alone fastener applicator, for hand use without a separate holder or alignment jig.
Advantageously, the cartridge preferably has two knurled or other grippable surfaces. The distal surface (closer to the fastener end) permits more accurate manual alignment when applying a fastener to an implant in a shallow wound, reducing the risk of cross-threading due to angular misalignment between the central axes of the implant and the cartridge, and allowing axial rotation by fingertip manipulation. The cartridge may also possess an integral, on-axis handle extension shaft facilitating application of the clamping fasteners in deep wounds and allowing a screwdriver-like hand tightening.
Systems of the invention may further include a holder or alignment guide having a centering body with distal end jaws adapted to nest onto and circumferentially grip the head of the implant, and thus align the centering body with the body of the anchor screw or the axis of the head structure. In these systems, the fastener cartridge fits within the alignment guide, where it is turned by hand or with a tool, applying the fasteners concentrically without risk of cross-threading. The alignment guide may be used with cartridges that hold a single fastener or a concentric pair of fasteners, or cartridges that hold multiple fasteners or multiple concentric fastener pairs.
In one embodiment, the alignment guide includes an elongated handle projecting to a side of the centering body, allowing a surgeon to manipulate the centering body into position on the implanted anchor within the surgical wound while holding the handle assembly at an offset position outside of the surgical wound. In another embodiment, the alignment guide has an axially centered handle. This embodiment includes a substantially cylindrical centering guide that is telescopically interfitted with another substantially cylindrical member, and having an enlarged torque grip for handling. This embodiment allows the alignment guide to be positioned on-axis while held by a surgeon""s hand sufficiently removed from the operative site. A second torque grip may be positioned closer to the fastener end to facilitate manipulation of the holder for actuating a clamping mechanism to align to and grip the anchor implant.
The alignment guide preferably includes distal end jaws which may be opened and then clamped about the head of the anchor assembly to effectively center the guide on the head such that the cartridge is held on-axis and does not cross-thread when rotated.